The isomerization of normal paraffins of low molecular weight is highly important in the oil industry in view of the particularly high octane number of the isoparaffins formed.
The modification of the regulations in the main industrial countries concerning quality standards of motor gasolines and the progressive cancellation of prior authorizations of using lead-containing additives induce producers to search for improved processes in view to produce lead-free motor gasolines of high octane number.
Processes for converting normal paraffins, having for example 4,5,6 or 7 carbon atoms per molecule, in particular n-paraffins of 5-6 carbon atoms per molecule, to a product containing a high proportion of isoparaffins, are particularly interesting.
By these processes, in particular, the octane number of light gasoline fractions, such for example as straight-run gasolines or those obtained by catalytic reforming, may be improved.
The mechanism of the hydroisomerization reaction is usually considered as a bifunctional mechanism for which catalysts comprising both acid sites and sites of hydrogenating/dehydrogenating function are preferred.
For about twenty years many papers have mentioned the use, in hydroisomerization processes, of catalysts essentially comprising more or less extensively modified zeolites, particularly mordenites, usually in acid form, associated with at least one metal from group VIII of the periodic classification of elements providing for the hydrogenating/dehydrogenating function.
The catalyst efficiency depends in particular on a good dispersion of the metal on mordenite in acid form. It is desirable to obtain the largest possible metal dispersion onto mordenite, so that a maximum of metal atoms be accessible to the reactants. The size of the metal crystallites must be low, preferably at most 10 Angstroms (10.times.10.sup.-10 m), and their initial distribution onto the freshly prepared catalyst (i.e. a catalyst which has not been contacted with hydrocarbons under isomerization conditions) must be as homogeneous as possible, mainly after regeneration of the at least partially deactivated catalyst.
In fact, the coke, whose formation is unavoidable during the isomerization reaction, deposits onto the catalyst, thus taking part in the total performance decrease of the catalyst which must then be regenerated so as to extend its total life time.
The conventional catalyst regeneration comprises a step of coke removal by combustion. In said step the catalyst is heated into a more or less diluted oxygen stream at a temperature of about 400.degree.-500.degree. C., so as to burn coke. Care must be taken, during this treatment, to avoid a more or less substantial surface loss of the metal particles, obviously resulting in a corresponding loss of catalyst activity. This sintering of the metal phase is well known in the art. Accordingly, special procedures for regenerating these catalysts have been developed.
A particular way of regenerating a platinum-containing zeolite catalyst is disclosed in U.S. Pat. No. 3,986,982. This regeneration consists of contacting the catalyst, after coke burning, with a gas mixture containing an inert gas, 0.5-20% by volume of oxygen and 5-500 ppm by volume of chlorine, either as chlorine, hydrochloric acid or organic chlorinated compound, then purging the catalyst so as to remove oxygen and residual chlorine, and finally reducing this catalyst under hydrogen stream at a temperature of 200.degree.-600.degree. C.
This method, although resulting in a clear improvement of the catalyst activity, does not provide for a good redispersion of platinum, and consequently does not result in an activity of the regenerated catalyst close to that of the fresh catalyst.